For long distance signal transmission through an optical fiber telecommunication line, it is necessary that the transmitted signals reach the receiving station at a level sufficient to allow their recognition and use. Since, in the propagation of the signals in an optical fiber, there is an attenuation of the signal light, and in order to obtain at the receiving station, which can be tens or hundreds of kilometers distant from the transmitting station, a signal of sufficient level, the signal originally introduced in the fiber must have the highest possible level.
However, the lasers used for the generation of the transmitted signal have somewhat limited power, not more than some mW, whereas more powerful lasers are not able to generate such signals. Thus, it becomes necessary to use power amplifiers which receive the signal generated by the laser and amplify it up to the desired level.
It is known that optical fibers having a core doped with particular substances, e.g. rare earth ions, provide stimulated output characteristics suitable for use as optical amplifiers.
In fact, such fibers can be supplied with a light source, called a "pumping source", at a particular wavelength corresponding to a peak of the absorption spectrum of the doping substance, which is able to bring the atoms of the doping substance in an excited energetic condition, or pumping band, from which the same atoms drop spontaneously, in very short time, to a laser output condition where they remain for a relatively longer time.
When a fiber having a high number of atoms at the excited state in the emission level is transited by a light signal having a wavelength corresponding to such laser emission state, the signal causes the transition of the excited atoms to a lower level with light emission having a wavelength the same as the wavelength of the signal. Therefore, a fiber of such kind can be used to obtain an amplification of an optical signal.
In particular, optical amplifiers using Erbium as laser-emission dopant are well known and make use of the fluorescence of the Erbium at around 1550 nm for the amplification of a signal in the fluorescence range, in which range the line fiber has the best transmission characteristics from the point of view of the attenuation of the signal.
Power optical amplifiers using fibers doped with Erbium have been described in "Proceeding ECOC (European Conference on Optical Communication) 1989", pages 42-43. Such article describes a power amplifier that uses fibers of silica doped with Germanium and Erbium and pumped with a Nd-YAG laser doubled in frequency to 532 nm.
However, such amplifier has a very low amplification efficiency, i.e. the ratio between the power of the transmission signal at the output and the supplied pumping power, lower than 20% which is very far from the maximum theoretical efficiency.
From U.S. patent application, Ser. No. 07/363,072 filed Jun. 8, 1989 and entitled "Optical Fibre Amplifier", fibers of the type doped with Al/Er are known. The dopant used to obtain the index refraction profile suitable to guide the light is Al.sub.2 O.sub.3 and the fibers are suitable for optical amplifiers pumped at 514.5 nm wavelength, e.g. with an Argon laser.
According to the last-mentioned patent application, the Al/Er doped fibers are preferable to the traditional Ge/Er fibers if pumped at a wavelength of 514.5 nm because they avoid absorption phenomenon in the excited state which occurs in the Ge/Er doped fibers at such wavelength.
In order to obtain a high amplification efficiency, on the other hand, it is convenient to use relatively high pumping wavelengths, and in particular, the wavelength of 980 nm is useful because the amplification efficiency in a power amplifier is substantially proportional to the pumping wavelength.